A turbofan gas turbine engine used for powering an aircraft in flight includes a fan assembly having a plurality of circumferentially spaced apart fan blades extending radially outwardly from a rotor disk. Ambient airflow is channeled between the blades and pressurized thereby for generating thrust for powering the aircraft in flight. A radially inner flowpath boundary for the airflow channeled between the blades is provided typically by integral platforms formed at the blade roots which circumferentially abut each other between adjacent fan blades. Since the platforms are formed integrally with the fan blades, the centrifugal loads therefrom during operation of the fan must be carried by the fan blade dovetails which requires that they be suitably large, which in turn requires a suitably large rotor disk for accommodating all of the centrifugal loads within acceptable stress limits. Additionally, for composite rotor blades it is impractical to integrally form the platforms with the blade itself.
In order to reduce the weight of the fan blades, it is known to provide discrete platforms independently joined to the rotor disk between adjacent fan blades. These separate platforms must have suitable strength for accommodating both centrifugal loads and inpact loads, such as those due to a bird strike, during operation. For example, in a wide chord fan blade configuration, the corresponding discrete platforms must have suitable strength in the axial direction to accommodate shear and bending stresses due to the centrifugal or impact loads. In one configuration, each of the platforms includes an axially extending stiffening rib which radially supports an outer flange defining the platform flowpath surface, and which has a complementary inner flange on its radially inner end. The transverse cross section, therefore, is in the form of a conventional I-beam which is a relatively rigid structure in both bending and shear between its two axial ends.
However, for larger-thrust fan assemblies having relatively long axial chords and relatively wide circumferential spacing therebetween, the platforms required to fill the space between adjacent blades are also relatively long in the axial direction and relatively wide in the circumferential direction. Using a conventional configuration such as the I-beam configuration, the resulting platform would be undesirably heavy and would require a correspondingly large rotor disk. The larger platform also has increased surface area which increases the exposure to and potential impact force resulting from a designed-for bird strike event. For a circumferentially wide platform, the top flange thereof would have a larger unsupported overhang extending from the center stiffening rib which decreases its ability to suitably accommodate the bird impact loading thereon.
Furthermore, conventional discrete platforms are typically joined to the rotor disk at both their forward and aft ends for accommodating both the centrifugal loads from the platform itself which are directed radially outwardly, and the bird impact loads which are directed radially inwardly and axially aft. Accordingly, not only must the platform itself have suitable strength, but the retention structures therefor must be suitably strong for accommodating both the centrifugal and bird impact loads.